In internal combustion engines, such as spark-ignition engines and diesel engines, (in particular, diesel engines), it is required to effectively reduce the generation amount of harmful substances, such as NOx and PM (hereinafter may be collectively referred to as “emission”), generated upon operation of the engines.
In order to reduce the emission generation amount, controlling the emission generation amount to a predetermined target value is effective. In a diesel engine, the generation amount of NOx has a strong correlation with the EGR ratio. Therefore, a control apparatus for an internal combustion engine (diesel engine) described in Japanese Patent Application Laid-Open (kokai) No. 2002-371893 estimates the amount of NOx generated from combustion, by use of an expanded Zeldovich mechanism, which is a typical known combustion model, from combustion temperature and gas-mixture concentration calculated on the basis of combustion pressure and intake-gas oxygen concentration, which are detected by means of a cylinder interior pressure sensor and an intake-gas oxygen concentration sensor, respectively. Then, the control apparatus controls the EGR ratio, etc. such that the estimated NOx generation amount coincides with a predetermined target value.
Incidentally, it is known that when the EGR ratio is increased to reduce the NOx generation amount, the PM generation amount increases (in particular, in diesel engines). That is, the ratio between the NOx generation amount and the PM generation amount is determined by the EGR ratio, and when the EGR ratio is controlled only for reduction of the NOx generation amount, the PM generation amount increases. This is unfavorable from the viewpoint of effectively reducing the emission generation amount as a whole.
In other words, in order to effectively reduce the emission generation amount as a whole, a target value of the emission generation amount (accordingly, a target value of the EGR ratio) must be determined in consideration of the balance between the NOx generation amount and the PM generation amount.
In view of the above, in general, a combination of a steady-condition appropriate value of the NOx generation amount (steady-condition appropriate NOx generation amount) and a steady-condition appropriate value of the PM generation amount (steady-condition appropriate PM generation amount) under the present operating conditions of the engine (for example, fuel injection quantity, engine rotational speed, etc.) is used as a target value of the emission generation amount. The EGR ratio is controlled such hat the actual EGR ratio coincides with an EGR ratio target value corresponding to the emission generation amount target value.
Here, the steady-condition appropriate NOx generation amount and the steady-condition appropriate PM generation amount are an NOx generation amount and a PM generation amount which are previously determined such that their combination becomes optimal from the viewpoint of the balance between the NOx generation amount and the PM generation amount in the case where the engine is operated steady under present operating conditions (for example, fuel injection quantity, engine rotational speed, etc.).
Such steady-condition appropriate NOx generation amount and steady-condition appropriate PM generation amount can be obtained as follows. An experiment is performed so as to determine the steady-condition appropriate NOx generation amount and the steady-condition appropriate PM generation amount in a state where the engine is maintained at a certain steady operating condition. The experiment is repeated while the operation conditions (for example, fuel injection quantity, engine rotational speed, etc.) of the engine are changed in various ways.
However, in general, even when the engine is under the same operating conditions (for example, fuel injection quantity, engine rotational speed, etc.), the relation between the NOx generation amount and the PM generation amount changes depending on whether the engine is in steady operating condition or transient operating condition; e.g., a condition in which the rotational speed of the engine increases abruptly. In other words, even when the engine is under the same operating conditions (for example, fuel injection quantity, engine rotational speed, etc.), the ratio between the NOx generation amount and the PM generation amount for the same EGR ratio changes depending on whether the engine is in transient operating condition or steady operating condition.
Accordingly, in the case where the EGR ratio is controlled, while the steady-condition appropriate NOx generation amount and the steady-condition appropriate PM generation amount are used as a target value of the emission generation amount, when the engine is in transient operating condition, the balance between the NOx generation amount and the PM generation amount is lost, and as a result, there arises a problem in that the emission generation amount cannot be effectively reduced as a whole.